1. Field of the Invention
The invention relates to semiconductor wafer processing systems. More particularly, the invention relates to a method for providing pulsed plasma in a semiconductor wafer processing system.
2. Description of the Background Art
As structures fabricated upon semiconductor wafers are reduced in size, charging damage has become a serious problem. Charging damage generally occurs when structures being formed on a semiconductor wafer using a plasma enhanced process cause non-uniform charging of the structures such that a differential voltage is formed on the structures. Such a differential voltage can produce high currents or arcing in the structure that damages the structure. For example, a plasma etch process can damage the gate oxide of a transistor structure due to a Fowler-Nordheim current produced by structure charging.
Electron shading is one of the main contributors to structure charging. Electron shading is caused by the formation of structures having a dense line pattern having a high aspect ratio. Electron shading results from the isotropic movement of electrons in a process chamber during plasma processing versus the anisotropic movement of ions contained in the plasma. The electrons impact the sidewalls and other vertical surfaces of the structures and charge the structures. However, a high aspect ratio of these structures causes portions near the plasma to be impacted by more electrons than the xe2x80x9cdeepxe2x80x9d portions of the structure. As such, the xe2x80x9cshadingxe2x80x9d of the deep portions results in a voltage differential over the structure. Such structure charging may result from the use of any plasma process to process a wafer. As a result, many plasma processes are capable of causing topography dependent charging damage of structures on a semiconductor wafer. Topography dependent charging impacts many aspects of semiconductor wafer processing including: electron shading damage, notching, loss of profile control, aspect ratio dependent etching, etch stop, microloading, decreased photoresist selectivity, photoresist striations, and decreased etch rate.
Therefore, a need exists in the art for a method of reducing topography dependent charging.
The disadvantages associated with the prior art are overcome by the present invention of a method for plasma processing a semiconductor wafer using a pulsed RF power. In the method of the present invention, RF power is applied to a process gas to form a plasma in a chamber. The wafer is processed with the plasma using continuous RF power for a first period of time T1 (a first phase of processing) and with pulsed RF power a second period of time T2 (a second phase of processing). The time T1 includes, for example, an initial portion of a main etch process, while the time T2 includes a final portion of the main etch process and at least a portion of an over etch process. The pulsed power applied during time T2 minimizes damage to circuits formed on the wafer as a result of charging and greatly reduces electron shading and other undesirable effects of wafer processing with plasma. For certain types of plasma processes, the continuous power may be applied during the second period and the pulsed power may be applied during the first period. Also, pulsed power may be applied during multiple phases of a plasma process, e.g., continuous power, then pulsed power, then continuous power, then pulsed power.
The method of the present invention can be implemented as a program code stored in a computer readable storage medium. The program code is executed by a computer to control a semiconductor wafer processing system during wafer processing. The semiconductor wafer processing system comprises a processing chamber and a RF power supply that is controlled in accordance with the method of the present invention.